Shift current photovoltaic effect in a ferroelectric charge-transfer complex.

Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of the electronic origin. Here, we report the photovoltaic effect in an organic molecular crystal tetrathiafulvalene-p-chloranil with a large ferroelectric polarization mostly induced by the intermolecular charge transfer. We observe a fairly large zero-bias photocurrent with visible-light irradiation and switching of the current direction by the reversal of the polarization. Furthermore, we reveal that the travel distance of photocarriers exceeds 200 µm. These results unveil distinct features of the shift current and the potential application of ferroelectric organic molecular compounds for novel optoelectric devices.The bulk photovoltaics refers to an effect whereby electrons move directionally in non-centrosymmetric crystals upon light radiation. Here, Nakamura et al. observe this effect in a ferroelectric organic charge-transfer complex, which shows large diffusion distance of photogenerated electrons over 200 µm.

Metabolic pathway elucidation towards time- and dose-dependent electrophoretic screening of stable oxidative phenolic compounds.

This study demonstrates an untested link between model phenolic compounds and the formation/electrophoretic separation of stable urinary metabolites. Sterically encumbered carbonyl groups were examined, and mass determination was used to confirm the presence and stability of two oxidative metabolites of pentachlorophenol: tetrachloro-1,2-benzoquinone and tetrachloro-1,4-dihydroquinone. Subsequently, baseline resolved separation of pentachlorophenol and the two oxidative metabolites was demonstrated under the following conditions: 75 mM sodium tetraborate buffer (pH = 8.5) with 5 % methanol and 50 mM SDS, +10.0 kV running voltage, injection time = 5.0 s, effective capillary length = 55 cm, and run temperature = 20 °C. Results not only provide key metabolic inferences for pentachlorophenol, they also exhibit improvements in the ability to separate and detect changes in urinary metabolites in response to phenolic-related exposure.

Sequestration of a highly reactive intermediate in an evolving pathway for degradation of pentachlorophenol.

Microbes in contaminated environments often evolve new metabolic pathways for detoxification or degradation of pollutants. In some cases, intermediates in newly evolved pathways are more toxic than the initial compound. The initial step in the degradation of pentachlorophenol by Sphingobium chlorophenolicum generates a particularly reactive intermediate; tetrachlorobenzoquinone (TCBQ) is a potent alkylating agent that reacts with cellular thiols at a diffusion-controlled rate. TCBQ reductase (PcpD), an FMN- and NADH-dependent reductase, catalyzes the reduction of TCBQ to tetrachlorohydroquinone. In the presence of PcpD, TCBQ formed by pentachlorophenol hydroxylase (PcpB) is sequestered until it is reduced to the less toxic tetrachlorohydroquinone, protecting the bacterium from the toxic effects of TCBQ and maintaining flux through the pathway. The toxicity of TCBQ may have exerted selective pressure to maintain slow turnover of PcpB (0.02 s(-1)) so that a transient interaction between PcpB and PcpD can occur before TCBQ is released from the active site of PcpB.

Substituent-induced switch of the role of charge-transfer complexes in the Diels-Alder reactions of o-chloranil and styrenes.

Addition of p-substituted styrenes, XSty (X = H, Me, MeO, or Cl) to the solutions of o-chloranil, oCA, in dichloromethane resulted in the transient formation of the charge-transfer complexes, [XSty, oCA], followed by the Diels-Alder reaction. At low temperatures, these reactions led to formation of essentially pure endocycloadducts. As expected for the inverse-electron-demand Diels-Alder reaction, the rate constants of the cycloaddition rose with the increase of the donor strength. However, while facile cycloaddition took place in the neat mixtures of the o-chloranil with p-methyl, p-chloro-, or unsubstituted styrenes at low temperatures, a similar system involving the strongest MeOSty donor was surprisingly persistent. X-ray structural measurements and quantum-mechanical computations indicated that this anomaly is related to the fact that the diene/dienophile orientation in the charge-transfer [MeOSty, oCA] complex is opposite to that in the endocycloadduct and in the lowest-energy transition state leading to this isomer. Thus, the proceeding of the cycloaddition requires dissociation of the (dead-end) complex. For the systems involving the oCA diene and either the HSty, ClSty, or MeSty dienophile, the donor/acceptor arrangements in the charge-transfer complexes apparently are consistent with that in the corresponding products, and the formation of these complexes does not hinder the Diels-Alder reaction.

Spectrophotometric, Fourier transform infrared spectroscopic and theoretical studies of the charge-transfer complexes between methyldopa [(S)-2 amino-3-(3,4-dihydroxyphenyl)-2-methyl propanoic acid] and the acceptors (chloranilic acid, o-chloranil and dichlorodicyanobenzoquinone) in acetonitrile and their thermodynamic properties.

Methyldopa is a much used antihypertensive drug. It is the subject matter of study mostly for the determination and estimation of methyldopa in pharmaceutical properties. These considerations led us to study the charge-transfer interactions between methyldopa, a centrally acting antihypertensive agent of limited use with the known acceptors like o-chloranil (o-ClN), chloranilic acid (ClA) and dichlorodicyanobenzoquinone (DDQ). Methyldopa (MDP) formed beautifully colored complexes (having absorption maxima at 581 nm and 368 nm; 519 nm; 583.5 nm, 547 nm and 346 nm, respectively) with the acceptors mentioned before. The physico-chemical properties of the complexes were studied using UV-visible spectrophotometry and FTIR measurements. The composition, the accurate association constants and thermodynamics of the complexes were determined spectrophotometrically. Attempts were made to interpret the thermodynamics of complexes in terms of I(D)(V), E(A)(V) and hν(CT). Solid CT complexes between MDP+o-ClN, MDP+ClA and MDP+DDQ were prepared and FTIR spectra of the complexes were studied. The energies hν(CT) of the charge-transfer complexes and vertical ionization potential I(D)(V) of methyldopa were compared with the theoretical values of hν(CT) obtained from HOMO and LUMO of the donors and acceptors calculated using Density Function Theory utilizing different basis sets. The agreement between the results can be regarded to be reasonable. Oscillator strengths and dipole strengths of the complexes were determined theoretically and experimentally and the limitations of the calculations were outlined.

In situ coupled oxidation cycle catalyzed by highly active and reusable Pt-catalysts: dehydrogenative oxidation reactions in the presence of a catalytic amount of o-chloranil using molecular oxygen as the terminal oxidant.

An in situ coupled oxidation cycle that allows catalytic oxidation of a substrate with catalytic amounts of o-chloranil and novel reusable polymer-immobilized platinum nanocluster catalysts using molecular oxygen as the terminal oxidant was developed.

Ultrafast potential energy surface softening of one-dimensional organic conductors revealed by picosecond time-resolved Laue crystallography.

Time-resolved Laue crystallography has been employed to study the structural dynamics of a one-dimensional organic conductor (tetrathiafulvalene-p-chloranil) during photoexcitation in the regime of the neutral to ionic phase transition. Exciting this crystalline system with 800 nm 100 fs long optical pulses leads to ultrafast population of a structural intermediate as early as 50 ps after excitation with a lifetime of at least 10 ns. Starting from the neutral phase, this intermediate has been assigned as a precursor state toward the photoinduced population of the ionic phase. The observed intensity changes are significantly different from comparable equilibrium structures. The interpretation of this structural data is that the potential of this intermediate is being softened during its population in a dynamical process. The depopulation proceeds through thermal processes.

Experimental and theoretical studies of redox reactions of o-chloranil in aqueous solution.

The electrochemical reduction of o-chloranil (OCA) in aqueous solution has been studied experimentally and theoretically. The effects of temperature and pH on various thermodynamic parameters were studied by means of cyclic voltammetry. The pH dependence of the redox activity of OCA in aqueous solution at temperatures in the range 25-40 degrees C was used for the experimental determination of the standard reduction potentials of both the one-proton-two-electron (0.67 V) and two-proton-two-electron (0.79 V) reduction processes. The temperature dependency of the equilibrium constants of studied reactions has been used in order to determine enthalpy, entropy, and Gibbs energy changes of the reactions. It is found that the two studied redox reactions of OCA are strongly affected by solvation effects and controlled by entropy contributions to the Gibbs free energies. High-level ab initio calculations (G3 and CBS-QB3 using the CPCM solvation model) have been employed to calculate the reduction potentials of OCA in aqueous solution for the one-proton-two-electron (0.65 and 0.69 V at the respective levels of theory) and two-proton-two-electron (0.81 and 0.83 V) reactions, which are in excellent agreement with the corresponding experimental values. The acidic strength of the reduced form of OCA in aqueous solution, pK(a), has been also calculated (5.2) and is also in excellent agreement with the experimental value (5.0). The agreement mutually verifies the accuracy of experimental method and the validity of the mathematical model.

Determination of total retronecine esters-type hepatotoxic pyrrolizidine alkaloids in plant materials by pre-column derivatization high-performance liquid chromatography.

A pre-column derivatization high-performance liquid chromatography method with diode array detection was developed and validated to determine the total retronecine esters-type hepatotoxic pyrrolizidine alkaloids (RET-HPAs) in herbs. The RET-HPAs reacted with o-chloranil in methanolic solution heated for 3 h, and an oxidative derivative was produced that could be detected at a maximal absorption of 223 nm. The analysis was performed using a C18 column with an isocratic elution of methanol and aqueous 0.01% triethylamine (adjusted to pH 4 with formic acid), and the detection was carried out with DAD at 223 nm. The validation of the method included linearity, sensitivity, recovery and stability. It showed a good linear regression (r(2) > 0.9900) in the range of 2.5-250 microM with a limit of detection (S/N = 3) of 0.5 microM. The method provided desirable repeatability with overall intra- and inter-day variations of less than 4.6%. The obtained recoveries for both of the extraction and derivatization process were between 94.6 and 100.7% (n = 3).

Versatile Friedel-Crafts-type alkylation of benzene derivatives using a molybdenum complex/ortho-chloranil catalytic system.

A variety of molybdenum complexes catalyze Friedel-Crafts-type alkylation reactions of benzene derivatives with alkenes and alcohols in the presence of an organic oxidant, o-chloranil. The utilization of [Mo(CO)(6)] and two equivalents of o-chloranil catalytically furnished the hydroarylation product of norbornene with p-xylene at 80 degrees C, whereas [Cr(CO)(6)] and [W(CO)(6)] failed to catalyze the same reaction, thus indicating the importance of the molybdenum source. The best results were obtained when a molybdenum(II) complex [CpMoCl(CO)(3)] (Cp=cyclopentadienyl) was used as a precatalyst. The hydroarylation reactions also took place with styrenes, cyclohexenes, and 1-hexene as olefin substrates. The electrophilic-substitution mechanism was proposed on the basis of the ortho/para selectivities and the Markovnikov selectivities observed for the hydroarylation products. Our hypothesis was further corroborated by the fact that in the presence of the [CpMoCl(CO)(3)]/o-chloranil catalytic system, secondary, benzylic, or allylic alcohols participated in the alkylation of benzenes with similar selectivities.

Photoprocesses of chloro-substituted p-benzoquinones.

The photochemistry of chloro-(ClBQ), dichloro-(2,5- and 2,6-Cl 2BQ), and trichloro-1,4-benzoquinone (Cl 3BQ) was studied in aqueous solution and/or in mixtures with acetonitrile. Final products are the corresponding hydroquinones (QH 2s) and 2-hydroxy-1,4-benzoquinones (QOHs). Three transients were detected by UV-vis absorption spectroscopy. The triplet state appears within the 20 ns 248 nm pulse and is converted within 0.1-1 micros into a photohydrate (HI aq). HI aq which is spectroscopically and kinetically separated from the triplet state decays within 5 ms, whereas the anion of the hydroxyquinone (QO (-)) grows in at ca. 500 nm in the 0.1-1 s time range. The proton formation and decay kinetics within 0.1-10 micros were observed by transient conductivity in the course of the reaction of the triplet state with water en route to HI aq at pH 4-9. Formation of QO (-) results in a permanent conductance. The efficient photoconversion of Cl n BQs at low concentrations (<0.2 mM) into QH 2s and HOQs is due to a redox reaction of Q with rearranged HI aq. The quantum yield of photoconversion at lambda irr = 254 nm is 0.8-1.2 for ClBQ or Cl 2BQs in aqueous acetonitrile and smaller (0.4) for Cl 3BQ. The yield of semiquinone radical ( (*)QH/Q (*-)) of Cl n BQs ( n = 1-4) in acetonitrile-water (1:1) is low (<20%) at low substrate concentration but is significantly increased upon addition of an H-atom donor, for example, 2-propanol. Other mechanisms involving (*)QH/Q (*-) radicals, such as quenching of the triplet state at enhanced Cl n BQ concentrations and H-atom abstraction from an organic solvent in mixtures with water, have also to be considered.

Oxidative dimer produced from a 2,3,4-trihydroxybenzoic ester.

The DPPH radical-scavenging abilities of the naturally occurring phenolic acid, 2,3,4-trihydroxybenzoic acid, and its methyl ester were evaluated. Both compounds in acetonitrile scavenged as many as four radicals compared to three or fewer radical consumption in acetone or ethanol. Only the ester showed relatively high ability in methanol. Oxidation with o-chloranil in acetonitrile resulted in methyl 2,3,4-trihydroxybenzoate giving a novel benzocoumarin-type dimer, its chemical structure being confirmed by spectroscopic evidence. The formation of this dimer might partly account for the higher radical-scavenging efficiency of the ester in acetonitrile or methanol.

The first structural study on a cyclic tricoordinate phosphorochloridite and a pentacoordinate phosphorane based on 1,2,3,5-protected myo-inositol--a new conformation of 1,3,2-dioxaphosphorinane ring.

Treatment of the phosphoramidite [myo-C(6)H(6)-2-[OC(O)Ph]-1,3,5-(O(3)CH)-4,6-(O(2)P-NH-i-Pr)] with o-chloranil affords the first example of inositol-based pentacoordinate phosphorane [myo-C(6)H(6)-2-[OC(O)Ph]-1,3,5-(O(3)CH)-4,6-(O(2)P-NH-i-Pr)(1,2-O(2)C(6)Cl(4))] (9) (X-ray structure) with a trigonal bipyramidal geometry at phosphorus. The six-membered 1,3,2-dioxaphosphorinane ring with the inositol residue has an unusual boat conformation in 9 which is quite different from that found in unrestrained rings investigated before, but is similar to that of its P(III) chloro precursor [myo-C(6)H(6)-2-[OC(O)Ph]-1,3,5-(O(3)CH)-4,6-(O(2)PCl)] (X-ray structure). Also, a convenient and chromatography-free procedure for the protected myo-inositol derivative [myo-C(6)H(6)-2-[OC(O)Ph]-1,3,5-(O(3)CH)-4,6-(OH)(2)] is reported.

Inhibition of jack bean urease by tetrachloro-o-benzoquinone and tetrachloro-p-benzoquinone.

Tetrachloro-o-benzoquinone (TCoBQ) and tetrachloro-p-benzoquinone (TCpBQ) were studied as inhibitors of jack bean urease in 20 mM phosphate buffer, pH 7.0, 1 mM EDTA, 25 degrees C. The mechanisms of inhibition were evaluated by analysis of the progress curves obtained with two procedures: the reaction initiated by addition of the enzyme and the reaction initiated by addition of the substrate after preincubation of the enzyme with the inhibitor. The obtained results were characteristic of slow-binding inhibition. The effects of different inhibitor concentrations on the initial and steady-state velocities obeyed the relationships of two-step enzyme-inhibitor interaction, qualified as mechanism B. It was found that TCoBQ and TCpBQ are strong urease inhibitors. TCpBQ is more effective than TCoBQ with the overall inhibition constant of K(i)* = 4.5 x 10(-7) mM. The respective inhibition constant of TCoBQ was equal to: K(i)* = 2.4 x 10(-6) mM. The protective experiment proved that the urease active site is involved in the tetrachlorobenzoquinone inhibition process. High effectiveness of thiol protectors against inhibition by TCoBQ and TCpBQ indicates the strategic role of the active site sulfhydryl group in the blocking process. The stability of the complexes: urease-TCoBQ and urease-TCpBQ was tested in two ways: by dilution or addition of dithiothreitol. No recovery of urease activity bound in the urease-inhibitor complexes proves that the complexes are stable and strong.

Catalytic, enantioselective [4 + 2]-cycloadditions of ketene enolates and o-quinones: efficient entry to chiral, alpha-oxygenated carboxylic acid derivatives.

We report catalytic, enantioselective [4 + 2]-cycloadditions of o-quinones with ketene enolates (derived from readily available acid chlorides) using cinchona alkaloid derivatives as catalysts to produce products in high enantiomeric excess (ee) and good to excellent yields. The thermodynamic driving force for these reactions is due in part to the restoration of aromaticity to the products. The resulting chiral, bicycloadducts can be synthetically manipulated in a variety of useful ways, for example to provide a flexible synthesis of alpha-oxygenated carboxylic acid derivatives.

Spectroscopic and thermodynamic study of charge transfer complexes of cloxacillin sodium in aqueous ethanol medium.

Cloxacillin sodium has been shown to form charge transfer (CT) complexes of 1:1 stoichiometry with a number of electron acceptors in 50% (v/v) aqueous ethanol medium. From the trends in the CT absorption bands, the vertical ionization potential of the drug molecule (cloxacillin sodium) has been estimated to be 7.89 eV. The enthalpies and entropies of formation of two such complexes have been determined by estimating the formation constants spectrophotometrically at five different temperatures. The oscillator strengths and transition dipole moments of these complexes have been determined. It has further been noted that the reduction of o-chloranil by aqueous ethanol is completely inhibited by cloxacillin sodium, a phenomenon that makes the present study of formation equilibrium possible.

The effect of pressure on hydrogen transfer reactions with quinones.

The effect of pressure on the oxidation of hydroarenes 3-9 with 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ; 1 a) or o-chloranil (10), leading to the corresponding arenes, has been investigated. The activation volumes were determined from the pressure dependence of the rate constants of these reactions monitored by on-line UV/Vis spectroscopic measurements in an optical high-pressure cell (up to 3500 bar). The finding that they are highly negative and only moderately dependent on the solvent polarity (DeltaV( not equal ) = -13 to -25 in MTBE and -15 to -29 cm(3) mol(-1) in MeCN/AcOEt, 1:1) rules out the formation of ionic species in the rate-determining step and is good evidence for a hydrogen atom transfer mechanism leading to a pair of radicals in the rate-determining step, as was also suggested by kinetic measurements, studies of kinetic isotope effects, and spin-trapping experiments. The strong pressure dependence of the kinetic deuterium isotope effect for the reaction of 9,10-dihydroanthracene 5/5-9,9,10,10-D(4) with DDQ (1 a) can be attributed to a tunneling component in the hydrogen transfer. In the case of formal 1,3-dienes and enes possessing two vicinal C--H bonds, which have to be cleaved during the dehydrogenation, a pericyclic hydrogen transfer has to considered as one mechanistic alternative. The comparison of the kinetic deuterium isotope effects determined for the oxidation of tetralin 9/9-1,1,4,4-D(4)/9-2,2,3,3-D(4)/9-D(12) either with DDQ (1 a) or with thymoquinone 1 c indicates that the reaction with DDQ (1 a) proceeds in a stepwise manner through hydrogen atom transfer, analogously to the oxidations of 1,4-dihydroarenes, whereas the reaction with thymoquinone 1 c is concerted, following the course of a pericyclic hydrogen transfer. The difference in the mechanistic courses of these two reactions may be explained by the effect of the CN and Cl substituents in 1 a, which stabilize a radical intermediate better than the alkyl groups in 1 c. The mechanistic conclusions are substantiated by DFT calculations.

Spectrophotometric study of molecular complex formation between o-chloranil and a series of methylated pyridines.

o-Chloranil has been shown to form 1:1 molecular complexes with pyridine and 2-, 3- and 4-picolines in CCl4 medium. Isosbestic points have been found but charge-transfer bands could not be detected. The formation constants of the complexes exhibit a very good linear free energy relationship from which the Hammett p parameter for the complexation reaction is found to be -3.67.

Characterization of metabolic activation of pentachlorophenol to quinones and semiquinones in rodent liver.

Pentachlorophenol (PCP), a widely used biocide, induces liver tumors in mice but not in rats. Metabolic activation of PCP to chlorinated quinones and semiquinones in liver cytosol from Sprague-Dawley rats and B6C3F1 mice was investigated in vitro (1) with microsomes in the presence of either beta-nicotinamide adenine dinucleotide phosphate (NADPH) or cumene hydroperoxide (CHP), (2) with CHP in the absence of microsomes, and (3) with horseradish peroxidase (HRP) and H2O2. Mono-S- and multi-S-substituted adducts of tetrachloro-1,4-benzoquinone (Cl4-1,4-BQ) and Cl4-1,2-BQ and their corresponding semiquinones [i.e. tetrachloro-1,4-benzosemiquinone (Cl4-1,4-SQ) and tetrachloro-1,2-benzosemiquinone (Cl4-1,2-SQ)] were measured by gas chromatography-mass spectrometry (GC-MS). Qualitatively, the metabolites of PCP were the same in both rats and mice for all activation systems. Induction of PCP metabolism by either 3MC or PB-treated microsomes was observed in NADPH- but not in CHP-supported systems. In rats, the amount of induction was comparable with either 3MC or PB. 3MC was a stronger inducer than PB in mice and also induced a greater amount of metabolism than in rats. This suggests that induction of specific P450 isozymes may play a role in the toxicity of PCP to mice. Both HRP/H2O2 and CHP led to production of the full spectrum of chlorinated quinones and semiquinones, confirming the direct oxidation of PCP. CHP (with or without microsomes) converted PCP into much greater quantities of quinones and semiquinones than did microsomal P450/NADPH or HRP/H2O2 in both species. This implies that, under conditions of oxidative stress, endogenous lipid hydroperoxides may increase PCP metabolism sufficiently to enhance the toxicity and carcinogenicity of PCP.

Charge-transfer complex formation between o-chloranil and a series of polynuclear aromatic hydrocarbons.

The equilibrium constants, enthalpies and entropies of formation of molecular electron donor-acceptor (EDA) complexes of o-chloranil with a series of aromatic hydrocarbons have been determined spectrophotometrically. Spectroscopic and thermodynamic aspects of these complexes have been analysed.